1. Field of the Invention:
This invention relates to polyester imide resins, and more specifically to novel polyester imide resins capable of providing insulated wires which are excellent in solderability, softening point, heat resistance and workability. The term "solderability" as used herein means the property of an insulating film or the like that is strippable with molten solder.
2. Description of the Related Art:
In recent years, remarkable achievements have been made in the size and weight reduction of electric components such as motors and transformers. These achievements have taken an important role for the size and weight reduction not only in the field of home electric appliances but also in the fields of automotive vehicles and aircrafts. On the other hand, there is a strong demand for improvements in the reliability of electric components.
From the foregoing viewpoint, there is an outstanding demand for the development a material which has excellent heat resistance and is usable as a coating material for insulated wired to be employed in electric components such as motors and transformers.
For the size and weight reduction of such components, it is also necessary to make wires thinner. Since insulated wires of such a reduced diameter are subjected to heavier loads compared to conventional insulated wires, they are obviously require to exhibit higher performance.
As a result, the heat resistance of insulating materials for insulated wires has been improved, leading to the development of thermally stable materials such as class F (155.degree. C.) glycerin-containing polyester imide resins, class H (180.degree. C.) polyester imide resins containing tris-(2-hydroxyethyl) isocyanurate (hereinafter abbreviated as "THEIC"), class H (180.degree. C.) THEIC-containing polyester amide-imide resins, class H (180.degree. C.) THEIC-containing polyester resins, class K (200.degree. C.) aromatic polyamide-imide resins and class M (220.degree. C.) polyimide resins.
Since insulated wires coated principally with these resins are used under severe conditions, such resins are required to have chemical resistance, solvent resistance, hydrolysis resistance and alkali resistance in addition to heat resistance.
Besides improvements in heat resistance and the like of insulating materials, manufacturers of electric components also wish to rationalize the production steps for the reduction of the production cost and to improve the performance of the insulating materials. As one example of such wishes, it is desired to reduce the man power required for the end stripping work of these insulated wires and also to automate the end stripping work.
Insulated wires making use of the above-described insulating materials however have excellent chemical resistance, so that the automation of the end stripping work of these insulated wires is prevented.
There are now various methods for performing the end stripping work, including (1) mechanical stripping, (2) stripping relying upon thermal decomposition, (3) chemical stripping and (4) stripping with molten solder. The stripping method making use of molten solder (4) is however most preferred in view of the time required for the stripping work, the prevention of damages to thin conductors, the possibility of continuous performance of the stripping work, etc.
Accordingly, there is a strong demand from manufacturers of electric components for the development of insulating materials capable of affording insulated wires which permit the end stripping work with molten solder, in other words, are solderable and have heat resistance rated as class F (155.degree. C.) to class H (180.degree. C.).
In order to meet the above demand, solderable polyester imide resins have been developed.
In the present field of art, the expression "solderable" means that when an insulated wire is dipped in a bath of heated and molten solder, its insulating film is decomposed and removed at the dipped section and the solder is also applied on the conductor to facilitate subsequent soldering. Accordingly, it should not be interpreted in such a way that direct soldering is feasible.
Upon soldering a number of insulated wires of a twisted structure, there is a recent tendency to perform their end stripping work in such a way that the insulated wires with their insulating films still applied thereon are dipped directly in a bath of molten solder so as to perform both removal of the insulating films and soldering at the same time. For this purpose, the insulating films must be removed as soon as possible, namely, immediately after their dipping in the bath of the molten solder. Needless to say, the shorter the dipping time in the bath of the molten solder, the better.
In the end stripping work making use of molten solder, any molten solder temperatures higher than 400.degree. C. leads to accelerated oxidation and deterioration of the molten solder and faster dissolution of copper as the conductor of the wires in the molten solder. The latter problem turns to the problem of thinning or diameter reduction of the insulated wires.
Although the above-mentioned conventional polyester imide resins having solderability have heat resistance of at least class F, they require a solder bath temperature of at least 450.degree. C. and a dipping time period of at least 10 seconds in order to completely decompose insulating films in the solder bath and to avoid the existence of any carbonized coatings left over on the conductors. Nevertheless, their softening points are as low as 290.degree.-300.degree. C.
There is hence a demand for the development of an insulating material which permits end stripping work without leaving any carbonized coatings on conductors even when the temperature of the solder bath is 450.degree. C. or lower and the dipping time is 10 seconds or shorter and which also has an excellent softening point.